Abstract
The adsorption of a series of atoms and small molecules and radicals (H, C, N, O, NH, OH, H2O, CH3, and NH3) on hexagonal crystalline and amorphous ice clusters were obtained via classical molecular dynamics and electronic structure methods. The geometry and binding energies were calculated using a QMHigh:QMLow hybrid method on model clusters. Several combination of basis sets, density functionals and semi-empirical methods were compared and tested against previous works. More accurate binding energies were also refined via single point Coupled Cluster calculations. Most species, except carbon atom, physisorb on the surface, leading to rather small binding energies. The carbon atom forms a COH2 molecule and in some cases leads to the formation of a COH-H3O+ complex. Amorphous ices are characterized by slightly stronger binding energies than the crystalline phase. A major result of this work is to also access the dispersion of the binding energies since a variety of adsorption sites is explored. The interaction energies thus obtained may serve to feed or refine astrochemical models. The present methodology could be easily extended to other types of surfaces and larger adsorbates.
Highlights
Astrochemistry has been an area of intense research in particular with the aim to elucidate the origin of the chemical diversity observed in space
Theoretical Binding Energies on ices binding energy and the frequency characterizing the motion of the adsorbed species in the potential energy well
The study of the crystalline structures was performed to benchmark the various DFT/ONIOM models employed in the present work
Summary
Astrochemistry has been an area of intense research in particular with the aim to elucidate the origin of the chemical diversity observed in space. In low UV-photon field regions, as in dense molecular clouds, energetic processes are suppressed and surface reactions of cold molecules become important. It has been demonstrated both experimentally and theoretically that dust grains catalyze the formation of some important molecules, such as H2, H2O, and CO2, organic molecules Activated surface reactions rarely occur over the activation energy barriers on ice mantles because of their low temperatures (∼ 10 K). For a reaction to occur on the grain, the molecular species needs to reside or diffuse at the grain surface. Most neutral species can physisorb onto grains through van der Waals interactions at the low temperature conditions of dense molecular clouds. There is a crucial need to improve the binding energies (BE’s) values in the models to better reproduce the astrophysical abundances
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